MOLECULAR STRUCTURE 83 



in this way proving insufficient, conceptions going in 

 further detail into the relative positions of the atoms were 

 developed as a study of configuration or stereochemistry. 

 We shall here discuss each in turn, and in a third section 

 the very peculiar phenomenon known as tautomerism. 



i. DETERMINATION OF CONSTITUTION. 



The study of constitution, then, refers to the mode of 

 connexion between the atoms. In it the molecule is sup- 

 posed motionless, so that the theory could at most only 

 represent the facts accurately at the absolute zero. 



Of the methods in question which allow of deter- 

 mining the mode of connexion, there are, of special 

 importance : 



A. Determination of constitution on the basis of the 

 valency of the elements combined. 



B. Determination of constitution from formation out of, 

 and conversion into compounds of known structure. 



It may be added that many determinations of consti- 

 tution are based on analogy, and therefore on the relations 

 between physical and chemical properties and structure. 

 But these will, according to the arrangement chosen, find 

 their natural place in Part III. 



A. Determination of Constitution on the Basis of the 

 Valency of the Elements combined. 



The notion of valency, on which this method is based, 

 must first be quite briefly considered in its origin and extent. 

 It is worth while then to glance at the binary hydrogen 

 compounds of known molecular weight, in which only one 

 atom of the other element occurs in the molecule, as : 



CH 4 , NH 3 , OH 2 , C1H, SiH 4 , PEL, SH 2J IH, &c. 



It appears then that never more than one atom of another 

 element is combined with a single atom of hydrogen. This 

 fact is expressed in the conception that hydrogen has only 



F 2, 



